The Financial Conduct Authority has written to the chief executives of Britain’s four largest banks over concerns a compensation scheme for victims of interest rate swap mis-selling is moving too slowly.

The FCA said in May it expected banks to deal with all claims for redress within six to 12 months, but admitted it had become “frustrated that they are all expecting to meet the lower end of the FCA expectations”.

Barclays, HSBC, Lloyds Banking Group and Royal Bank of Scotland have all signed up the redress scheme. The latest figures from the regulator show that while Barclay and HSBC have completed the basic checks on all their claims, RBS still has nearly 1,700 customers waiting to be assessed to see if they are eligible to join the process.

“Current trends indicate that the banks may not meet this timetable so the FCA have written to the chief executives of the four major banks to make our expectations clear and agree practical ways to speed the process up,” said the regulator in a statement.

Banks had promised to send out 1,000 compensation offers last month, but the latest figures from the regulator showed just 800 offer letters were sent out to customers in October.